Fuel cell powerplants produce electric power by electrochemically consuming a fuel and an oxidant in one or more electrochemical cells. The oxidant may be pure oxygen or a mixture of gases containing oxygen, such as air. The fuel may be hydrogen.
Each fuel cell generally has electrodes for receiving the gases, such as an anode electrode for fuel and a cathode electrode for an oxidant. The cathode electrode is spaced from the anode electrode and a matrix saturated with electrolyte is disposed between the electrodes.
Each electrode includes a substrate. The substrate has a catalyst layer disposed on the side of the substrate which faces the electrolyte matrix. In some instances, an electrolyte reservoir plate is on the other side of the substrate and is capable of providing electrolyte through small pores to the substrate. These electrolyte reservoir plates may have channels or passageways behind the substrate for carrying a reactant gas, such as gaseous fuel to the anode and gaseous oxidant to the cathode. For example, these channels might extend between parallel ribs on the substrate side of the electrolyte reservoir plate. A separator plate on the other side of the electrolyte reservoir plate provides a barrier to the transfer of electrolyte and prevents mixing of the fuel and oxidant gases in adjacent cells. Another acceptable construction is to have the electrode substrate act both as an electrolyte reservoir plate and as an electrode substrate with channels on the separator side of the substrate.
Generally, a stack of fuel cells and separator plates are used in performing the electrochemical reaction. As a result of the electrochemical reaction, the fuel cell stack produces electric power, a reactant product, and waste heat. A cooling system extends through the stack for removing the waste heat from the fuel cell stack. The cooling system has a coolant and conduits for the coolant. The conduits are disposed in cooler holders to form coolers within the stack. Heat is transferred by the cooler holders from the fuel cells to the conduits and from the conduits to the coolant.
The cooler holder must be electrically and thermally conductive and may be permeable to gas. An example of such a cooler holder is shown in U.S. Pat. No. 4,245,009 issued to Guthrie entitled "Porous Coolant Tube Holder for Fuel Cell Stack". Alternatively, the cooler holder might be impermeable to gas. An examole of such a cooler holder is shown in U.S. Pat. No. 3,990,913 issued to Tuschner entitled "Phosphoric Acid Heat Transfer Material". In Tuschner, the cooler holder serves the double function of cooler holder and separator plate.
Separator plates prevent the mixing of the fuel gas, such as hydrogen, disposed on one side of the plate, with an oxidant, such as air, disposed on the other side of the plate. Separator plates are, therefore, highly impermeable to gases such as hydrogen and highly electrically conductive to pass the electrical current through the fuel cell stack. In addition, separator plates must also tolerate the highly corrosive atmosphere formed by the electrolyte used in the fuel cell. One example of such an electrolyte is hot, phosphoric acid. In addition, separator plates, like cooler holders, must be strong, particularly in terms of flexural strength, which is a measure of the ability of the separator plate to withstand high pressure loads, differential thermal expansion of mating components, and numerous thermal cycles without cracking or breaking.
An example of a method for making separator plates for electrochemical cells is discussed in U.S. Pat. No. 4,360,485 issued to Emanuelson et al., the disclosure in which is hereby incorporated by reference. In this method, the separator plate is formed by molding and then graphitizing a mixture of preferably 50 percent high purity graphite powder and 50 percent carbonizable thermosetting phenolic resin. In particular, Emanuelson discusses forming a well blended mixture of the appropriate resin and graphite powder. The mixture is then distributed in a mold. The mold is compacted under pressure and temperature to melt and partially cure the resin and to form the plate.
Electrolyte reservoir layers, such as are commonly found in electrolyte reservoir plates and as electrode substrates have requirements that differ from those for a separator plate. For example, reservoir layers must accommodate volume changes in the electrolyte during fuel cell operation. Examples of such electrolyte reservoir layers are shown in commonly owned U.S. Pat. Nos. 3,779,811; 3,905,832; 4,035,551; 4,038,463; 4,064,207; 4,080,413; 4,064,322; 4,185,145; and 4,374,906.
Several of these patents show the electrolyte reservoir layer as an electrode substrate. In addition to accommodating changes in acid volume due to electrolyte evaporation and changes in operating conditions of the cell electrode, substrates must satisfy several other functional requirements. For example, the substrate must be a good electrical conductor, a good thermal conductor and have adequate structural strength and corrosion resistance. The substrate provides support to the catalyst layer and provides a means for the gaseous reactants to pass to the catalyst layer. Finally, the edges of the substrate are often required to function as a wet seal to prevent the escape of reactant gases and electrolyte from the cell.
This may be done in the manner described in U.S. Pat. No. 3,867,206 entitled "Wet Seal for Liquid Electrolyte Fuel Cells" issued to Trocciola et al. which is commonly owned with the present invention. Another example is shown in commonly owned U.S. Pat. No. 4,259,389 issued to Vine entitled "High Pressure-Low Porosity Wet Seal". As discussed in Vine, a seal may be formed in the edge seal region of a porous plate by using a powder filler to provide a denser packing to the region which reduces porosity. Nevertheless, this aporoach has not been widely adopted.
Another approach to forming edge seals is to increase the density of the edge region by compressing the edge region. Densified substrate edge seals are described in commonly owned U.S. Pat. Nos. 4,269,642 and 4,365,008. Experience has shown that the seal density and pore size that can be practically obtained limits the edge seal cross pressure (or, commonly called the bubble pressure) to 3-4 psi. This is lower than the 10 psi desired for a fuel cell stack that operates at 120 psia where pressure differentials between reactants can reach to 5-10 psid.
Accordingly, scientists and engineers are seeking to develop seals for porous plates of an electrochemical which can withstand the higher transient pressures associated with higher pressure fuel cells.